The present invention relates generally to acceleration responsive devices, and more specifically to an improved permanent magnet/moving coil rebalance system for linear accelerometers.
Acceleration responsive devices perform essential sensing functions in a wide variety of systems. As performance requirements and available technology have advanced, the demand has increased for sensors characterized by much improved sensitivity, stability, accuracy, linearity of response, reliability and ruggedness, in addition to fast reaction time and low cost. Implicit in the stability, sensitivity and response linearity requirements is a requirement that precision be maintained over a wide temperature range. The present state of the art is such that it has been difficult to achieve improvements in all of the foregoing characteristics simultaneously, or, in some instances, even to achieve improvement in one characteristic without adversely affecting another. Nevertheless, requirements exist, particularly in aircraft navigation and missile guidance systems, for a single acceleration sensor with superior performance in all of the noted areas.
One of the functions which must be provided for in an accelerometer is that of returning the acceleration sensing mass to its desired rest position after it has been displaced by an acceleration. This function will hereinafter be referred to as rebalancing the accelerometer. A variety of rebalance system variations have been devised and are presently employed. In the simplest, lower accuracy accelerometers, the resilience or spring constant of various materials has been relied on to produce rebalancing forces. Inertia has also been relied on to provide the rebalancing forces in certain types of accelerometers, such as the pendulous gyro accelerometer. Other more sophisticated and accurate accelerometers have employed permanent magnet/moving coil and electrostatic rebalance systems.
Permanent magnet/moving coil rebalance systems have shown considerable promise in simultaneously meeting the performance and low cost requirements. However, one feature of the permanent magnet/moving coil type of rebalance system that results in nonlinear response arises from the need to vary the coil current as the function of input acceleration in order to maintain balance. In conventional systems, this variable coil current produces a variable magnetomotive force which reenforces or opposed the magnetic field produced by the permanent magnet, and thus affects the magnetic flux density in the region in which the coil is located. Variations in the flux density result in response linearity errors in operation of the accelerometer, particularly at high input accelerations.
A known technique for reducing such errors is to provide two magnetic circuits arranged so that the coil current increases the flux density in one magnetic circuit and simultaneously decreases the flux density in the other circuit. Although such a system is theoretically effective in eliminating nonlinear response due to rebalance current in the coil, it requires magnetic circuits having identical temperature sensitivities. In addition, the magnetic circuits otherwise must be substantially identical for this arrangement to be effective. As a practical matter, it is difficult to produce two magnetic circuits which are adequately matched over a wide temperature range. Further, it has been found difficult to provide adequate compensation for nonlinear response produced by the rebalance current in conventional permanent magnet/moving coil rebalance systems.
A further problem which must be dealt with in achieving a satisfactory rebalance system for high performance accelerometers involves the relationship between the mechanical null position of the sensing mass suspension system and the electrical null position of the pickoff system. Precision over a wide temperature range dictates minimum changes in null bias and scale factor with changes in temperature. Null bias is dependent on both elastic restraint along the input axis and displacement between the mechanical and electrical null positions. Stability is predominantly the result of a stable relationship between the mechanical and electrical nulls.
Lack of null bias stability can result from physical movement of the suspension or pickoff null position, or electrical changes in the pickoff or its associated circuitry. Obviously, any interface between the suspension and pickoff systems is a potential source of change in the null bias. Change in null bias in a pendulous accelerometer is usually also accompanied by a change in alignment of motion of the sensing mass relative to the input axis. These problems may be minimized by providing a stable and linear (non-pendulous) suspension system and a stable pickoff system, and by minimizing interfaces between the suspension and pickoff systems.
The applicant has avoided the above-described problems by providing a unique single permanent magnet/double moving coil rebalance system with a variable capacitance pickoff in which common elements are used both to suspend the sensing mass and to serve as moveable plates of position sensing capacitors. This arrangement eliminates any net effect of coil generated magnetomotive forces on the field produced by the permanent magnet, and eliminates all interfaces between the suspension and pickoff systems. In accordance with these features, linear accelerometers employing the applicant's rebalance system design have been found to provide performance and reliability heretofore unavailable from simple low cost apparatus.